1. Field of the Invention
The present invention relates to a charged-particle distribution measuring apparatus for use in an ion mixing system, an ion implantation system or the like. More particularly, the present invention concerns a charged-particle distribution measuring apparatus which is capable of measuring the two-dimensional distribution of a large-current beam over a large area with a high resolution without deflecting the beam.
2. Description of the Related Art
In a large-current ion implantation over the large area of, for example, a semiconductor, it is essential to provide a uniform ion beam over the target area.
The current density distribution of this ion beam is conventionally measured by measuring the luminescence of a fluorescent material irradiated with the ion beam. However, this method has disadvantages in that highly accurate measurement is difficult and because the ion beam deteriorates and consumes the fluorescent material.
Another technique for measuring the current distribution in an electron beam is the wire beam probe method which has been proposed by, for example, J G Siekman et al
"New Microphobe Techniques For Measuring The Current Distribution In An Electron Beam Used For Welding", Journal of Physics 1975 Vol. 8. In that method the current distribution in an ion beam is measured by measuring a current that flows in a thin metal wire while the wire is translated and rotated. However, in this method, secondary electrons are discharged from the metal wire during the ion radiation, limiting this ion current distribution measurement method to a relative current density distribution.
A charged-particle distribution measuring apparatus with a secondary electron restricting electrode incorporated therein so that it can measure the absolute quantity of an ion beam is also known. FIG. 1A is a schematic view of such a conventional charged-particle distribution measuring apparatus which is proposed in, for example, "Electron/Ion Beam Handbook" on page 275. FIG. 1B is a cross-sectional view of the beam current detection portion (hereinafter referred to as a Faraday cage) of that charged-particle distribution measuring apparatus. In this apparatus, an incident beam restricting plate 1, a secondary electron restricting electrode 2, and cylindrical collector electrodes 3 are accommodated in a vacuum vessel 7. The incident beam restricting plate (particle passing member) 1 has a plurality of through-holes 1a formed therein. The secondary electron restricting electrode (a recoil particle trapping member) 2 has the same number of through holes 2a as are in beam restricting plate 1. The through-holes 2a are aligned with the corresponding through-holes 1a. The cylindrical collector electrodes (particle trapping members) 3 are disposed behind the individual through-holes 1a and 2a. An insulating plate 4 defines the positional arrangement of the plurality of collector electrodes 3 while electrically isolating them from each other. The collector electrodes 3 are fixed to the insulating plate 4 by means of measuring terminal screws 5. A conductor 8a is connected to the secondary electron restricting electrode 2 so that a negative potential can be applied thereto from a power source (not shown). Measuring wires 8b are connected to the measuring terminal screws 5. The other ends of the measuring wires 8b are connected to a current measuring device (not shown).
The operation of the charged particle measuring apparatus arranged in the manner described above will be described below. When the incident beam restricting plate 1 is irradiated with a beam of charged particles 6 emitted from a charged particle source (not shown), some of the charged particles strike the incident beam restricting plate 1 and others pass through the through-holes 1a. The charged particles which have passed through the through-holes holes 1a also pass through the through-holes 2a formed in the secondary electron restricting electrode 2, and then strike the collector electrodes 3. The charged particles striking the collector electrodes 3 are measured by a current measuring device (not shown) located outside of the vacuum vessel 7 through the measuring terminal screws 5 and the measuring wires 8b. The current distribution in the charged particle beam 6 over the incident beam restricting plate 1 fixedly disposed in a two-dimensional plane is measured by measuring the current values of the individual collector electrodes 3 by ammeters and then comparing the values obtained.
The conventional charged-particle distribution measuring apparatus arranged in the manner described above has a disadvantage in that the resolution of the apparatus in the two-dimensional space is limited by the intervals between the adjacent holes formed in the incident beam restricting plate 1. It is impossible to improve the resolution because of the mechanical restrictions.